Hygiena - Bigger is not always better

It has been known for >60 years that swabbing recovers only a small proportion of the total population of bacteria from surfaces yet many investigators expect precision and accuracy that neither the sample nor the detection method can produce.

Previous articles discussed some of the limitations of environmental surface testing and how these limitations can be mitigated to provide clear actionable results. Variability in the enumeration of low level contamination can be countered with appropriate analysis and binning of results into pass/caution/fail ranges. Delays in sample collection, transport, storage, and testing can be avoided with well-designed testing procedures informed by microbiological principles. However, some of the issues of testing environmental surfaces are poorly understood and remain unchallenged e.g. sample collection methods. In this article, we will look at some of the issues that affect the recovery and measurement of bacteria from surfaces, and how different sampling methods compare in performance.

The common sampling methods used are a standard cotton swab, wetted with ringers, resuspended in media. More recently large sponge (or cloth) swabs wetted with ringers and resuspended in media have been widely adopted. An alternative that avoids the need for resuspension of the sample is the use of self-contained devices that provide both the sample collection device and detection material whereby 100% of the sample collected is tested. Bigger is not always better.

The two key factors related to the sample collection method are:

Pick-up efficiency - getting the bacteria off the surface and onto the swab/sponge/cloth (sampling device) and

Release efficiency - resuspending the bacteria from the sampling device into the media used for the enumeration test

The pick-up efficiency of a method depends on the sample area covered, the pressure applied to the surface, the "stickiness" of the wetting agent and sampling device, the texture and type of surface being sampled, and the degree of attachment of bacteria to the surface.

Moore and Griffiths (2002) studied coliform enumeration from surfaces using different swabbing and detection methods. The surprising conclusion was that sponge swabs were significantly less efficient than all other methods tested. The limit of detection for traditional swabs and alternative self-contained swabs on inoculated wet surfaces was 1 - 3 cfu/cm2 whereas it was 200 - 900 cfu/cm2 for the sponge method. On dry surfaces where viability is markedly reduced, the limit of detection for traditional swab and sponge swabs was 104 - 105 cfu/cm2 whereas that of self-contained swab tests was 102 - 104 cfu/cm2.

The release efficiency had the greatest effect on swab sensitivity for environmental surface sampling. This appears to be due to resuspension of bacteria that are trapped in or adhere to the matrix of the sampling device such that not all the bacteria collected are available for enumeration by the test method. Other researchers commented that sponges are very absorbent and have a large internal surface area that repeated compression of the sponge only exacerbates the problem because the released bacteria are quickly reabsorbed and retained.

This would suggest that a self-contained swabbing system will provide more representative results than swab and sponge based methods requiring sample resuspension. It also begs the question as to the importance of enumerating bacteria from surfaces.